1. Field of the Invention
The invention relates to gas separation processes and more particularly the separation and recovery (or disposal) of perfluorocompound gases from a gas mixture. Especially, the invention relates to the concentrating of low concentration gas mixtures of perfluorocompound gases such as those present in the effluent of a semiconductor manufacturing process, particularly the etching and cleaning steps.
2. Related Art
The semiconductor industry is now using extensively perfluorocompounds such as CF4, C2F6, C3F8, C4F10, CHF3, SF6, NF3, and the like, in the semiconductor manufacturing processes involving gases, particularly in the various etching steps of the semiconductor manufacturing processes as well as in the chamber cleaning step of the manufacturing process. Those perfluorocompound gases are used either pure or diluted, for example with air or nitrogen or other inert gas or in admixture with other perfluorocompound gases or other carrier gases (for example inert gases). All of those gases do not necessarily react with other species during the manufacturing processes: accordingly, when reactors are cleaned or evacuated to carry out another step of the manufacturing process, the effluent gases or gas mixtures should not be vented, even if they are largely diluted with air or any other gas such as inert gas. Most of the perfluorocompounds (also called PFCs) have lifetimes measured in thousands of years in the atmosphere and are also strong infrared absorbers. In the xe2x80x9cGlobal Warming Symposiumxe2x80x9d held on Jun. 7-8, 1994, in Dallas, Tex., USA, carbon tetrafluoride (CF4), hexafluoroethane (C2F6), nitrogen trifluoride (NF3), and sulfur hexafluoride (SF6) have been identified as greenhouse gases of concern to the semiconductor industry.
In the presentation made at this symposium by Michael T. Mocella and entitled xe2x80x9cPerfluorocompound Emission Reduction From Semiconductor Processing Tools: An Overview Of Options And Strategiesxe2x80x9d, the various possible strategies to control emission of these gases in the atmosphere were explained.
Apart from process replacement by non PFCs, several methods are already known or under development.
chemical-thermal decomposition with various activated metals wherein the spent bed material must be disposed. It is presently considered as commercially unproven even if it is under promising development.
combustion-based decomposition process (or chemical-thermal process) using a flame to supply both the thermal energy and the reactants for the decomposition. There are some safety issues associated with the hydrogen or natural gas fuels used and all the PFCs will produce HF as a combustion product (if the temperature is high enough), whose emissions are also of concern and must be dealt with also. High temperatures may also be generated using a resistance heater.
plasma-based decomposition process which involves the use of a plasma such as coupled radio frequency systems to partially decompose C2F6, with over 90% decomposition of C2F6. However, such systems are not yet commercially proven. Oxygen is usually needed to drive the decomposition to non PFC products with, however, the generation of HF which needs to be thereafter managed.
recovery process wherein the PFCs are recovered instead of being destroyed in order to be recycled. This kind of process is of a great interest because it is considered as the xe2x80x9cgreenestxe2x80x9d one. Different schemes, according to the author, are possible xe2x80x9cbased on combinations of adsorption or low temperature trapping of PFCsxe2x80x9d. There are, however, several challenges such as dealing with the large amount of nitrogen associated with the pump operation, the close boiling points of CF4 and NF3, the mixing of various process streams and/or possible reactions with adsorbents. While recycle is suggested, there are obvious questions about recycling such mixtures.
Another combustion system for destroying high nitrogen content effluent gas streams comprising PFCs is disclosed in the article entitled xe2x80x9cVector Technology""s Phoenix Combustorxe2x80x9d by Larry Anderson presented at the same symposium Jun. 7-8, 1994. This abatement system also uses a gas flame (using natural gas or hydrogen with air), which leads then to the same problem of HF generation and further destruction (plus the generation of NOx, CO2 inherent to any combustion process).
In the article presented at the same symposium by ATandT Microelectronics and Novapure Corporation and entitled xe2x80x9cPFC Concentration and Recyclexe2x80x9d, the authors acknowledge the advantages of the recovery processes which avoid production of carbon dioxide, NOx and HF (compared to combustion processes). Briefly, this process is disclosed as the use of a dual bed adsorber (activated carbon), wherein one of the beds is in the adsorption mode, while the second bed is regenerated: the PFCs are adsorbed on the carbon sieves while the xe2x80x9ccarrierxe2x80x9d gases, such as nitrogen, hydrogen are not adsorbed and are vented to the exhaust system. When the system is switched on the second adsorber, then the first one is evacuated using a vacuum pump, then the effluent is recompressed and the PFC gas mixture is recovered. One of the issues not yet resolved with such a system is that CF4, which is non polar, is not readily adsorbed by the carbon sieve and is then rejected with the vent gases. Also, any adsorption system is very sensitive to moisture and any trace of water has to be removed from the feed flow.
It is also known from U.S. Pat. No. 5,281,255 to use membranes made of rubbery polymers such as poly dimethyl siloxane or certain particular polymers such as a substituted polyacethylene (having a low glass transition temperature), to recover condensable organic components having a boiling point higher than xe2x88x9250xc2x0 C., essentially hydrocarbons (CH4, C2H6, and the like), said hydrocarbons having the property of permeating through said membranes much faster than air, and then recovering on the permeate side of the membrane said hydrocarbons. The permeate (hydrocarbons) is then recovered at either substantially atmospheric pressure or lower pressure while the non-permeate (e.g. air) is still at the original pressure of the feed stream but is vented, and all of the pressure energy of the feed stream is lost.
Also, it is disclosed in U.S. Pat. No. 5,051,114, a selectively permeable membrane formed from an amorphous polymer of perfluoro 2xe2x80x942 dimethyl 1-3-dioxole which is usable for separation of hydrocarbons or chlorofluorocarbons from, for example, air. Such a particular membrane apparently permeates oxygen and nitrogen faster than hydrocarbons and chlorofluorocarbons which can be recovered unexpectedly on the non-permeate side of the membrane, contrary to all of the membranes, including those disclosed in U.S. Pat. Nos. 4,553,983 and 5,281,255. In the ""114 patent; there is also disclosed a mixture of the amorphous polymer of perfluoro 2xe2x80x942 dimethyl 1-3 dioxole and polytetrafluoroethylene. All these perfluoro polymers are known to be resistant to most of the harmful chlorofluorocarbons and hydrocarbons which make them commercially suitable for such separation. However, this membrane is not currently available and there is no indication in this patent whether or not such a membrane is suitable for separation of PFCs from air or nitrogen particularly at low concentrations of PFCs in carrier gases, and at widely varying feed flow conditions.
There is still presently a need for a xe2x80x9cgreenxe2x80x9d process for concentration and/or recovery of PFCs from a gaseous stream, which can be used with a feed flow comprising or saturated with, moisture, which can handle safely the PFCs recovery and/or concentration even with important or extreme variations of flows and/or concentration of PFCs in the feed stream, which does not produce hydrofluoric acid (HF) as a residue from the destruction of the PFCs (in addition to the possible HF content of the feed).
It has now been unexpectedly found that effluent gases, for example, from a semiconductor manufacturing process, which comprise perfluorocompounds can be treated efficiently by using certain, preferably hollow fiber, membranes which permeate much faster the xe2x80x9ccarrier gasesxe2x80x9d of the effluent gas mixture, such as air, nitrogen, oxygen, argon and/or helium, than the PFCs of the gas mixture which are then recovered on the non-permeate side of the membrane.
Preferred membranes are glassy polymeric membranes, more preferably asymmetric or composite (with an asymmetric outer layer) membranes. Preferably, these glassy polymeric membranes do not include perfluorinated membranes. However, the glassy polymeric membranes used in accordance with the invention can comprise a layer, including a posttreatment layer as disclosed in U.S. Ser. No. 08/138,309 filed Oct. 21, 1993(now abandoned), and which is incorporated herein by reference, made of a fluorinated polymer such as polytetrafluoroethylene, amorphous perfluoro 2xe2x80x942 dimethyl 1-3 dioxide, and the like.
One aspect of the invention relates to a process to recover at least one perfluorocompound gas from a gas mixture, comprising the steps of:
a) providing a gas mixture comprising at least one perfluorocompound gas and at least one carrier gas, the gas mixture being at a predetermined pressure;
b) providing at least one glassy polymer membrane having a feed side and a permeate side, the membrane being permeable to the at least one carrier gas and being non-permeable to the at least one perfluorocompound gaseous species;
c) contacting the feed side of the at least one membrane with the gas mixture;
d) withdrawing from the feed side of the membrane as a first non-permeate stream at a pressure which is substantially equal to the predetermined pressure, a concentrated gas mixture comprising essentially the at least one perfluorocompound gas, and
e) withdrawing from the permeate side of the at least one membrane as a permeate stream a depleted gas mixture consisting essentially of the at least one carrier gas.
According to another aspect, the invention also relates to a process to recover a perfluorocompound gas or gas mixture from a gas mixture flowing out from a semiconductor manufacturing process, comprising the steps of pretreating the gas mixture to substantially remove most of the harmful components (gas, particles, and the like) to the membrane and delivering a pretreated gas mixture, providing at least one glassy polymer membrane having a feed side and a permeate side, contacting the feed side of the membrane with the pretreated gas mixture at a first pressure, withdrawing the perfluorocompound gas or gas mixture from the feed side of the membrane at a pressure which is substantially equal to the first pressure and withdrawing a residue gas at a second pressure which is lower than the first pressure from the permeate side of the membrane. The semiconductor manufacturing process using PFCs may be selected from etching processes including oxide, metal and dielectric; deposition processes including silicon CVD, tungsten backetching, dry chamber cleaning, and the like.
As some of the glassy membranes are sensitive to certain products which may be harmful for them, i.e. which may destroy or plug them quickly, it is preferred to scrub the gas mixture prior to sending it on the membrane. Preferably any kind of species which is present in the feed flow stream which may harm the membrane is removed by the scrubber means, including any harmful gaseous HF, NH3, WF6, O3, BCl3, and any corrosive species, also any pyrophoric species including silicon hydrides such as SiH4, and any particulates having average diameter greater than about 20 micrometers, and any oil mists. Indeed, it is preferred that compressors used in the methods and systems of the invention be sealed and oil-free.
One preferred aspect of the invention relates to a process to recover at least one perfluorocompound gas or gas mixture, comprising the steps of:
a) providing a glassy polymer membrane having a feed side and a permeate side;
b) providing a gas mixture at a first pressure comprising at least one perfluorocompound gaseous species, at least one harmful species for the membrane, and at least one carrier gas;
c) treating said gas mixture in scrubber means in order to substantially remove harmful species for said membrane and reduce the concentration of said harmful species to an acceptable level for said membrane and receiving a scrubbed gas mixture at a second pressure;
d) contacting the feed side of said membrane with said scrubbed gas mixture at substantially said second pressure or at a higher pressure;
e) withdrawing a concentrated gas mixture comprising a higher concentration of the at least one perfluorocompound gas than in the scrubbed gas mixture, from the feed side of the membrane as a non-permeate stream at a pressure which is substantially equal to said second pressure; and
f) withdrawing a depleted gas or gas mixture from the permeate side of said membrane as a permeate stream which is enriched in a carrier gas and depleted in the at least one perfluorocompound at a third pressure.
According to a preferred aspect of the invention, after concentrating the PFCs with a membrane, the various PFCs are separated from each other, by well known methods per se, such as selective condensation or adsorption in order to recover either separate PFCs or mixtures of PFCs having close boiling points. According to another aspect of the invention, the PFCs gas mixture is concentrated again, for example, with a second membrane, or the PFCs gas mixture is stored or recycled in the process (with or without additional treatment).
Other preferred process and system aspects of the invention include provision of a vacuum pump, heat exchanger, compressor, or cryogenic pump in order that the PFC gas mixture may be compressed, at least partially liquefied, and stored for future use. Another feature of the invention includes the provision of a process step where the PFC gas mixture is concentrated using a plurality of membranes arranged in series, with the possibility of the concentrated PFC gas mixture from each membrane unit being capable of use as a sweep gas of the permeate side of any one of or all of the membrane units in the series. A further aspect of the invention is the provision of a PFC gas mixture surge tank prior to the PFCs being recycled into the semiconductor manufacturing process, or prior to being routed to storage.
Another aspect of the invention is a semiconductor manufacturing system comprising:
a) at least one reactor chamber adapted to receive perfluorocompound gases, carrier gases, and the like, the reactor chamber having a reactor effluent gas conduit attached thereto;
b) at least one glassy polymer membrane separation unit having a feed side and a permeate side, the membrane being permeable to at least one carrier gas and being substantially non-permeable to at least one perfluorocompound gas, the membrane unit connected to the reactor chamber via the reactor effluent conduit, the membrane unit having a permeate vent conduit and a non-permeate conduit, the latter adapted to direct at least a portion of a perfluorocompound containing non-permeate stream from the said membrane unit to the reactor chamber. Preferred systems in accordance with the invention include provision of pretreatment and/or post-treatment means, such as dry or wet, (or both) scrubbers, thermal decomposers, catalytic decomposers, plasma gas decomposers and various filters as herein disclosed, prior to the reactor effluent stream entering the membrane unit. Also as herein disclosed, a plurality of membrane units may be arranged in series, either with or without provision of sweep gas of non-permeate on the permeate side of one or all membranes. Further preferred embodiments of systems of the invention included a damper or surge tank in the non-permeate conduit (i.e. between the first or plurality of membrane units and the reactor chamber); and the provision of a compressor, heat exchanger, cryogenic pump or vacuum pump on one or more of the non-permeate, PFC enriched stream(s), allowing the PFC enriched stream(s) to be stored in liquid form for future use. Also preferred are appropriate valves which allow the damper or surge tank, and the compressor for creating the liquid PFC mixture, to be bypassed, as explained more fully herein.
Preferred processes and systems of the invention include operating one or more of the membrane units at a constant concentration set-point for the PFC concentration in the non-permeate stream from each membrane unit. In this preferred system and process, the set-point concentration of the PFC in the non-permeate stream from each succeeding PFC membrane separation unit would of course be higher than the immediately preceding one. Appropriate sensors can be inserted into the non-permeate effluent conduit from each membrane unit to continuously or non-continuously analyze for PFC concentration, or, samples may be taken periodically or continuously from the non-permeate effluent from each membrane unit, which may be sent to dedicated analyzers either on-site or off-site. This information is preferably then forwarded to a process controller which may control for example the pressure of the feed to each membrane unit, temperature, flow, and the like. Also, when discussing the use of a sweep gas arrangement, the sweep gas may either be controlled via an open loop or a closed loop arrangement.
Another preferred system and process embodiment of the present invention includes the recycle of the permeate stream of either the first or succeeding stages of the membrane units (in other words, the carrier gas and other process gases are recycled). The carrier gases may be recycled directly to the reactor chambers, or may be delivered to heat exchangers, compressors, and the like to reduce them to liquid form for storage or future use. A recycle membrane may be provided, functioning to separate carrier gases from process gases.
Other preferred processes and systems of the invention are those wherein a waste stream from a pretreatment step for the gas mixture emanating from the semiconductor process is used to generate one or more perfluorocompounds or other chemicals, which may then be purified for use in the semiconductor process, or other chemical processes, as more specifically described in assignee""s copending application Ser. No. 08/666,694 filed Jun. 14, 1996, which is incorporated herein by reference.
Still other preferred processes and systems in accordance with the invention are those wherein one or more non-permeate streams is post-treated to remove non-perfluorocompounds. Post-treatment methods include those previously mentioned as suitable for pretreatment of the feed gas to the membrane.
Another aspect of the invention is a method of recovery of a relatively pure PFC stream from a vent stream from one or more gas cabinets, tube trailers, clean rooms, or the like using a membrane unit as described herein.